The present invention relates to the field of steam generators such as those used to produce steam for electric power or for use in industrial applications and, more particularly, to a fluid velocity measurement device for measuring the fluid velocity in a generating bank tube attached to a drum of a boiler, and which is also capable of determining the direction of flow in the generating bank tube.
Conventional steam generators used in the production of steam for industrial power plants utilize what is known as a generating bank to produce steam for the industrial process or steam turbine. FIG. 1 shows a section through such a generating bank (10). The generating bank (10), as is well known in the art, generally comprises an upper steam drum (12) and a lower "mud" drum (14). Connected therebetween are a plurality of generating bank tubes (16) which are in fluidic communication with the upper steam drum (12) and lower mud drum (14). The generating bank tubes (16) are arranged in sections across the width of the steam generator so that the flow of combustion gases generated by the combustion process flow across the generating bank tubes in the direction of the arrow shown in FIG. 1. The generating bank (10) is initially filled with water such that the lower mud drum (14), the generating bank tubes (16) and approximately the lower half of the upper steam drum (12) are filled with water. The combustion gases, being at an elevated temperature with respect to the water in the generating bank (10), transfer heat to the generating bank tubes (16) while passing across the generating bank (10). The heat raises the temperature of the water and eventually begins to turn a portion of the water into steam. As shown in FIG. 1, the generating bank tubes (16) can be arranged in several groups or sections designated front, middle and rear sections (18, 20, 22). Sections (18, 20, 22) are separated by cavities (24, 26) which would normally be provided with sootblowers (not shown) used to clean the generating bank tubes (16). The generating bank tubes in section (18), being exposed to the highest temperature combustion gases, will begin to generate steam before the tubes in sections (20) and (22). The steam, having a lower density than that of the water, will begin to rise in section (18) upwards into the steam drum (12). As heating by the combustion gases continues, a naturally circulating upflow of a steam water mixture and downflow of water will begin to occur in the generating bank (10). The front section (18) will tend to flow upwards, while the rearmost section (22) will tend to flow in a downward direction. The generating bank tubes (16) comprising the center section (20) will begin to flow either upwardly or downwardly depending upon their heat absorption from the combustion gases. The fluid within the downflowing sections usually consists of subcooled water. However, some downflowing sections can operate with a two-phase mixture of steam and water. The fluid within all of the upflowing sections is a two-phase mixture of water and steam which must be separated. Separation of the steam from the water takes place in the upper steam drum (12) by means of centrifugal cyclone separators (28) and primary and secondary steam scrubbers (30, 32). The steam is discharged from the upper steam drum (12) through one or more outlet steam connections (34) for use by the industrial process or by a steam turbine (not shown). The water separated from the steam water mixture is discharged from the bottom of the centrifugal cyclone separators (28) and returned to the downflowing generating bank tubes (16).
Variations in the heat transfer across the various sections (18, 20, 22) of the generating bank (10) will cause upsets in the flow of the circulating two-phase mixture of water and steam. Further, it is theoretically possible for some of the tubes (16) in the generating bank (10) to flow in an upward direction at certain levels of the steam production and to flow in an opposite direction at other levels of steam production. In order to more fully understand this phenomena, it is necessary to investigate the flow within an individual generating bank tube (16) while the steam generator is in operation. Any apparatus for this purpose however, must take into account the environmental conditions within which the generating bank tubes (16) must operate. For example, many industrial steam generators using a two-drum generating bank (10) as shown in FIG. 1 are used in the soda or Kraft process recovery setting. In a process recovery boiler, extreme caution must be utilized to prevent the mixing of water in the generating bank (10) with the black liquor produced in the process. This is because the mixing of water and the black liquor creates a highly explosive reaction which can cause catastrophic damage to the steam generator and operating personnel. Thus, while it might be convenient to install some type of measuring device on an easily accessible portion of the generating bank tube (16), i.e., on the tube (16) but in the gas stream, such an installation might be considered dangerous in view of these concerns.
It has thus become desirable to develop an apparatus for measuring the velocity of a fluid in a generating bank tube that avoids these problems, and which can also be used to determine the direction of flow of the fluid therein.